The present invention relates to mutant algae strains having novel photosynthetic traits, to methods of generating, identifying and/or isolating such mutants and to genes encoding proteins that regulate photosynthesis.
In large scale open algal growth systems, cultures must be grown at reasonably high culture densities and depths in the region of 20-30 cm. This culturing environment provides significant self-shading, ensuring that each cell experiences only a low average irradiance level. The predominant photo-physiological status of an algal cell under these conditions is the low light-acclimated state, which is characterized by a relatively large auxiliary light harvesting antenna system associated with the photosynthetic reaction centers. However, a larger light harvesting antenna in each individual cell exacerbates the self-shading of the culture, leading to an even lower average irradiance level, which prompts further increases in the antenna size of the algal cells in response. The overall result is a culture with very poor light penetration, ensuring that the majority of the open growth system is in darkness. Furthermore, the large and efficient light harvesting antenna drives saturation of photosynthesis at relatively low light intensities. Therefore in the surface layer of the ponds, where light is available, a significant portion of the incident light is in excess of the light required to drive maximum photosynthetic rates. This excess irradiance dissipates through thermal channels and is lost as heat. The light use efficiency in open growth systems is very low and it has been suggested that up to 80% of photosynthetic active irradiance, incident upon the pond surface, is lost as heat.
Thus, the low light acclimation response decreases the overall light use efficiency of a pond culture by increasing self-shading and lowering the saturating irradiance level for photosynthesis. Prior methods for decreasing light harvesting antenna size in algae have focused solely on the antenna, targeting the biosynthesis of light harvesting polypeptides directly, or reducing their assembly or function indirectly by disrupting chlorophyll biosynthesis, protein translational control, or protein localization mechanisms. As a result, the reduced-pigment strains obtained are often imbalanced in light harvesting, electron transport, and carbon fixation, which can adversely affect culture productivity.
Most photoautotrophs acclimate to differing levels of irradiance in order to maximize light capture under light limited conditions or to avoid the potentially deleterious effects of harvesting excitation energy in excess under high irradiance. The most obvious feature of the acclimation response to irradiance is a change in the level of pigmentation, typically associated with changes in the abundance of the auxiliary light harvesting antenna. Acclimation to irradiance is, however, a largely pleiotropic response, involving changes in composition and function at multiple levels within the photosynthetic machinery and throughout the organism. The regulation of acclimation to irradiance in oxygenic photoautotrophs is poorly defined and a greater understanding of the underlying regulatory network may enable the beneficial manipulation of the composition and function of the photosynthetic machinery.